A data extractor of the type referred to is useful, for example, in cases where a microprocessor checks on the performance of a PCM coupling network inserted between a set of incoming lines and a set of outgoing lines each carrying a group of channels in a respective frame. For this purpose it is necessary to compare the data appearing in a given time slot on an incoming line with those appearing in a corresponding time slot on an outgoing line temporarily coupled thereto. Since the intervening coupling network generally introduces a certain delay, normally amounting to a small number of frame periods, an input-side data extractor and an output-side data extractor must operate in a correlated manner with the same relative delay.